You are absolutely deluded, if not stupid, if you think that a worldwide collection of software engineers who can't write operating systems or applications without security holes, can then turn around and suddenly write virtualization layers without security holes.

It should be mentioned that most uses of virtualization is for resource utilization and not security. Instead of buying 10 individual servers, 1 larger server can be purchased and "carved up" into 10 virtual machines.

Also, Mainframes have had virtual machines for decades and have been secure to my knowledge, unlike Xen, KVM, etc., but this may also be due to hardware.

In software, since 1972 with IBM's VM/370. In hardware since 1975 with Amdahl's Multiple Domain Facility (MDF).

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...and have been secure to my knowledge, unlike Xen, KVM, etc., but this may also be due to hardware...

Security through hardware isolation and performance improvement, were the two key selling points for MDF. IBM's competitive response in the 1980s, Logical Partitioning (LPAR), offered (and still offers) similar capabilities.